8,024 research outputs found
Seismic Constraints on Interior Solar Convection
We constrain the velocity spectral distribution of global-scale solar
convective cells at depth using techniques of local helioseismology. We
calibrate the sensitivity of helioseismic waves to large-scale convective cells
in the interior by analyzing simulations of waves propagating through a
velocity snapshot of global solar convection via methods of time-distance
helioseismology. Applying identical analysis techniques to observations of the
Sun, we are able to bound from above the magnitudes of solar convective cells
as a function of spatial convective scale. We find that convection at a depth
of with spatial extent , where is the
spherical harmonic degree, comprise weak flow systems, on the order of 15 m/s
or less. Convective features deeper than are more difficult
to image due to the rapidly decreasing sensitivity of helioseismic waves.Comment: accepted, ApJ Letters, 5 figures, 10 pages (in this version
A robust extension to the triple plane pressure mode matching method by filtering convective perturbations
Time-periodic CFD simulations are widely used to investigate turbomachinery
components. The triple-plane pressure mode matching method (TPP) developed by
Ovenden and Rienstra extracts the acoustic part in such simulations. Experience
shows that this method is subject to significant errors when the amplitude of
pseudo-sound is high compared to sound. Pseudo-sound are unsteady pressure
fluctuations with a convective character. The presented extension to the TPP
improves the splitting between acoustics and the rest of the unsteady flow
field. The method is simple: i) the acoustic eigenmodes are analytically
determined for a uniform mean flow as in the original TPP; ii) the suggested
model for convective pressure perturbations uses the convective wavenumber as
axial wavenumber and the same orthogonal radial shape functions as for the
acoustic modes. The reliability is demonstrated on the simulation data of a
low-pressure fan. As acoustic and convective perturbations are separated, the
accuracy of the results increases close to sources, allowing a reduction of the
computational costs by shortening the simulation domain. The extended method is
as robust as the original one--giving the same results for the acoustic modes
in absence of convective perturbations.Comment: Accepted 15-05-11 by International Journal of Aeroacoustics to be
published in the special issue focusing on turbomachinery aeroacoustic
Validating Forward Modeling and Inversions of Helioseismic Holography Measurements
Here we use synthetic data to explore the performance of forward models and
inverse methods for helioseismic holography. Specifically, this work presents
the first comprehensive test of inverse modeling for flows using
lateral-vantage (deep-focus) holography. We derive sensitivity functions in the
Born approximation. We then use these sensitivity functions in a series of
forward models and inversions of flows from a publicly available
magnetohydrodynamic quiet-Sun simulation. The forward travel times computed
using the kernels generally compare favorably with measurements obtained by
applying holography, in a lateral-vantage configuration, on a 15-hour time
series of artificial Dopplergrams extracted from the simulation. Inversions for
the horizontal flow components are able to reproduce the flows in the upper 3Mm
of the domain, but are compromised by noise at greater depths.Comment: accepted for publication by the Astrophysical
A New Way to Make Waves
I describe a new algorithm for solving nonlinear wave equations. In this
approach, evolution takes place on characteristic hypersurfaces. The algorithm
is directly applicable to electromagnetic, Yang-Mills and gravitational fields
and other systems described by second differential order hyperbolic equations.
The basic ideas should also be applicable to hydrodynamics. It is an especially
accurate and efficient way for simulating waves in regions where the
characteristics are well behaved. A prime application of the algorithm is to
Cauchy-characteristic matching, in which this new approach is matched to a
standard Cauchy evolution to obtain a global solution. In a model problem of a
nonlinear wave, this proves to be more accurate and efficient than any other
present method of assigning Cauchy outer boundary conditions. The approach was
developed to compute the gravitational wave signal produced by collisions of
two black holes. An application to colliding black holes is presented.Comment: In Proceeding of CIMENICS 2000, The Vth International Congress on
Numerical Methods in Engineering and Applied Science (Puerto La Cruz,
Venezuela, March 2000
FISH: A 3D parallel MHD code for astrophysical applications
FISH is a fast and simple ideal magneto-hydrodynamics code that scales to ~10
000 processes for a Cartesian computational domain of ~1000^3 cells. The
simplicity of FISH has been achieved by the rigorous application of the
operator splitting technique, while second order accuracy is maintained by the
symmetric ordering of the operators. Between directional sweeps, the
three-dimensional data is rotated in memory so that the sweep is always
performed in a cache-efficient way along the direction of contiguous memory.
Hence, the code only requires a one-dimensional description of the conservation
equations to be solved. This approach also enable an elegant novel
parallelisation of the code that is based on persistent communications with MPI
for cubic domain decomposition on machines with distributed memory. This scheme
is then combined with an additional OpenMP parallelisation of different sweeps
that can take advantage of clusters of shared memory. We document the detailed
implementation of a second order TVD advection scheme based on flux
reconstruction. The magnetic fields are evolved by a constrained transport
scheme. We show that the subtraction of a simple estimate of the hydrostatic
gradient from the total gradients can significantly reduce the dissipation of
the advection scheme in simulations of gravitationally bound hydrostatic
objects. Through its simplicity and efficiency, FISH is as well-suited for
hydrodynamics classes as for large-scale astrophysical simulations on
high-performance computer clusters. In preparation for the release of a public
version, we demonstrate the performance of FISH in a suite of astrophysically
orientated test cases.Comment: 27 pages, 11 figure
Dispersion behavior of torsional guided waves in a small diameter steel gas pipe
Condition monitoring of gas pipes has been an important issue for gas companies. Failure to accurately identify condition of gas pipes result in numerous problems. Also, producing a condition monitoring system for buried pipelines is challenging. Small pipes (with diameters less than 50 mm) are considered here as most of the literature focuses on larger pipes. Guided wave theory will be introduced alongside a numerical simulation of the relevant dispersion curves of the system. This paper investigates the feasibility of using torsional guided waves for inspecting defects in buried pipes with small diameters. The pipes are assumed to be lossless and hence the effect of attenuation is ignored in the calculations. Upon finding the theoretical guided wave characteristics, experiments were conducted to see if the aim could be achieved in a realistic scenario. A steel pipe with a diameter of 34 mm and wall thickness of 5.5 mm is considered. High reverberation levels at high frequency propagations due to mode conversion are studied. Having only a limited number of transducers could be a reason for high reverberation at high frequencies
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